The invention relates to improvements in paper making machines, and more particularly to improvements in dewatering a web by thermal means which permits the machine to be run at high speeds.
In the continual striving for increased speed of operation in paper making machines, speeds above 3000 feet per minute in existing machines have encountered difficulty in the occurrence of flutter and breaks in the dryer section. One of the reasons is that the web is insufficiently dried and does not have sufficient strength to carry across the open draws necessary in a standard dryer where the web is threaded in a serpentine path back and forth between dryer drums. It has been discovered that if the web is dried to on the order of 50% bone dry, it will have the strength and stability to withstand an open draw between the dryer rolls. It is impractical to attain this dryness in a press section.
It is accordingly an object of the present invention to provide a high speed paper making machine for the production of fibrous paper web, particularly such as newsprint, which can operate at high speeds on the order of above 3000 feet per minute without encountering breaks or other damage to the web at the beginning of the dryer section.
In accomplishing the foregoing objective in accordance with the principles of the present invention, the web is carried on a first dryer run between the press and standard dryer section being continuously supported so that no flutter or wrinkling can occur. During this first dryer run, the web is dried from about 40% bone dry as it is received from the press section and leaves this run on the order of about 50% bone dry at which time it will have strength and stability to withstand an open draw in passing through the regular dryer drum section. The long continuous support on the run is provided in the form of an endless belt. Difficulties have been encountered in such support in providing stability to the belt, accommodating evaporation of the moisture from the web on the belt, imparting thermal energy at a sufficient temperature to cause meaningful evaporation from the web during the time it is supported and providing engagement between the web and the belt without generating air pockets or floating of the web such as caused by inadvertently capturing air between the web and the belt at high operating speeds.
It is accordingly a further object of the present invention to provide a mechanism which drives the web between the press section and dryer drum section by providing a long continuous support for the web and providing for heated air impingement on the web during its term of long continuous support.
A further object of the invention is to provide a support means for the belt which provides a long continuous support for the web which is capable of stable operation for long periods of time which eliminates pockets of air beneath the web, permits imparting thermal energy to the web without damage to the belt and accommodates satisfactory high speed transfer to the belt from the press section and from the belt to the dryer drums at the end of the long continuous supported run.
The invention provides a continuous looped belt of porous material capable of withstanding the high temperatures of the heated air, either inherently or by being provided with a heat resistant coating on the upper surface and a roll with a suction gland on the lead end for transfer of the web onto the belt without inducing a cushion of air beneath the web, and a heated air generating means for directing a blanket of heated drying air continuously onto the upper surface of the web while it is carried on the belt support. The belt is supported over a long slightly arcuate path on successive porous rolls with suction boxes therebetween. These suction "boxes" might more appropriately be characterized as suction zones within a single suction box extending the length of the first dryer run under the support belt. The porous rolls themselves define the sides of these zones with the ends being constructed of sheet metal. There are no tops since the belt is supported between successive porous rolls over each zone. The rolls are perforate and have suction ventilating glands therein so that air is removed beneath the porous belt. Between the spaced parallel rolls are the suction zones with curved edge surfaces. At the end of the long continuous run is a roll within the belt, preferably an ordinary heated dryer drum roll, over which the belt and web pass immediately before the web is threaded onto a felt to pass through the dryer drum section.
Other objects, advantages and features as well as equivalent structures and methods which are intended to be covered herein, will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments in the specification, claims, and drawings, in which: